Nonvolatile semiconductor memory device with redundancy circuit

ABSTRACT

A nonvolatile semiconductor memory device for use as a flash EEPROM includes a plurality of sectors each comprising a plurality of main memory cell regions each composed of a matrix of nonvolatile memory cells and at least one redundant memory cell region composed of a matrix of nonvolatile memory cells. When one of said nonvolatile memory cells in any one of the sectors is found defective and is selected by addressing, it is replaced with one of the nonvolatile memory cells in the redundant memory cell region.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor memory device, and more particularly to a nonvolatile semiconductor memory device known as EEPROM (electrically erasable programmable read-only memory).

2. Description of the Prior Art

FIG. 1 of the accompanying drawings shows, in block form, memory cells and associated circuit components of a flash EEPROM which can erase all data stored in memory cells at one time. The circuit arrangement shown in FIG. 1 corresponds to one bit of an output signal. If the flash EEPROM is of an 8-bit output configuration, it has eight circuits each identical to that shown in FIG. 1 which are arranged parallel to each other.

The flash EEPROM has terminals PP connected to an external voltage supply Vpp for supplying the high voltage required when the EEPROM writes or erases data. The flash EEPROM also has a plurality of column address lines Y₁₁, . . . , Y₁, Y_(s21), . . . , Y_(s2j) for indicating the row address of a memory cell, and a plurality of row address lines X₁, . . . , X_(k) for indicating the row address of a memory cell. In write mode, a data input line D_(IN) supplies a memory cell which has been selected by a column address line and row address line with data "0" that indicates a high voltage of VPG for actually writing data, or with data "1" that indicates a voltage of 0 V for writing no data. A signal ER is of a high level "H" in erase mode and of a low level "L" in other modes.

An N-channel enhancement MOSFET (hereinafter referred to as "NE-MOSFET") Q_(Y1W) for writing data has a gate controlled by the data input line D_(IN). The flash EEPROM also includes a plurality of NE-MOSFETs Q_(YS211), . . . , Q_(YS21j), Q_(Y111), . . . , Q_(Y1l), . . . , Q_(Y1jl) for selecting row addresses. Memory cells M_(111l), . . . , M_(lk1l) have respective drains connected in common to a digit line D_(11l), respective sources connected in common to a common source terminal CS, and respective gates connected respectively to the row address lines X₁, . . . , X_(k). Memory cells M₁₁₁₁, . . . , M_(1k11) have respective drains connected in common to a digit line D₁₁₁, respective sources connected in common to the common source terminal CS, and respective gates connected respectively to the row address lines X₁, . . . , X_(k). Memory cells M_(11jl), . . . , M_(1kjl) have respective drains connected in common to a digit line D_(1j1), respective sources connected in common to the common source terminal CS, and respective gates connected respectively to the row address lines X₁, . . . , X_(k). Memory cells M_(11jl), . . . , M_(1kjl) have respective drains connected in common to a digit line D_(1jl), respective sources connected in common to the common source terminal CS, and respective gates connected respectively to the row address lines X₁, . . . , X_(k). The memory cells M_(111l), . . . , M_(1k11), M_(111l), . . . , M_(1k1l) are arranged as a memory cell matrix MX_(1l) having an output node N_(1l), and the memory cells M_(1ljl), . . . , M_(1kjl), M_(11jl), . . . , M_(1kjl) are arranged as a memory cell matrix MX_(1j) having an output node N_(1j).

In read mode, a sensing amplifier SA1 serves to detect whether a memory cell which has been selected by a row address line and a column address line stores "0" (i.e., the memory cell has written data) or "1" (i.e., the memory cell has no written data or has erased data). The sensing amplifier SA1 can amplify a change in the voltage at a data bus SC₁. The sensing amplifier SA1 produces an output signal S_(out1) indicative of the detected result. A source switch SS applies a voltage of 0 V to the common source terminal CS in write and read modes, and an erase voltage V_(pp) to the common source terminal CS in erase mode.

The following table shows voltages at the signal lines and nodes at the time the memory cell M₁₁₁₁ is selected by row and column address lines in write, erase, and read modes:

    ______________________________________                                         Node                      YS.sub.21                                            Mode  PP     X.sub.1                                                                               Y.sub.11                                                                             (Y.sub.21)                                                                           D.sub.IN                                                                             D.sub.111                                                                             CS                                ______________________________________                                         Data                                                                           "0"                                   VPG -                                    Write 12 V   12 V   12 V  12 V  VPG   VTN    0 V                               Date                                                                           "1"                                                                            Erase 12 V   0 V    0 V   0 V   0 V   Float- 12 V                                                                    ing                                      Read  5 V    5 V    5 V   5 V   0 V   1.5 V  0 V                               ______________________________________                                    

Operation of the flash EEPROM in the above three modes will be briefly described below with reference to FIG. 1 and the above table.

(1) Write mode

A voltage Vpp of 12 V is applied to the terminals PP, a voltage of 12 V to the row address line X₁, a voltage of 0 V to the other unselected row address lines, a voltage of 12 V to the column address lines Y₁₁, Y_(S21), a voltage of 0 V to the other unselected column address lines, and a voltage of 0 V to the common source line CS.

When the data to be written is "0," the high voltage VPG which is determined according to the writing characteristics of the memory cells (VPG<Vpp) is applied to the data input line D_(IN). The voltage [VPG-VTN] (VTN is the threshold voltage of all the NE-MOSFETs) is applied to the drain of the memory cell M_(111l) over the digit line D₁₁₁, the voltage Vpp =12 V to the gate thereof, and the voltage of 0 V to the source thereof. Therefore, a channel current flows through the memory cell M_(111l) injecting hot electrons into a floating gate FG_(111l) of the memory cell M_(111l). As a result, the threshold voltage of the memory cell M_(111l) increases to a value higher than its reading voltage of 5 V.

When the data to be written is "1," a voltage of 0 V is applied to the data input line D_(IN), rendering the NE-MOSFET Q_(Y1W) nonconductive, and no high voltage is applied to the drain of the memory cell M_(111l) over the digit line D₁₁₁. Therefore, no data is written in the memory cell M_(111l).

(2) Erase mode

A voltage Vpp of 12 V is applied to the terminal PP, and a voltage of 0 V is applied to all the column address lines and all the row address lines. When the signal ER supplied to the source switch SS is of a high level ("H"), the voltage applied to the common source line CS rises from 0 V to Vpp and is balanced at Vpp. Since the common source line CS is shared by the sources of all the memory cells M_(111l) -M_(1kjl), a high electric field is applied between the gates and sources of all the memory cells M_(111l) -M_(1kjl). Electric charges stored in the floating gates of the memory cells M_(111l) -M_(1kjl) are discharged into the common source line CS. The threshold voltage of all the memory cells M_(111l) -M_(1kjl) is lowered, and the reading voltage thereof is also lowered.

(3) Read mode

A voltage VCC of 5 V is applied to the terminal PP, a voltage of 5 V to the row address line X₁, a voltage of 0 V to the other unselected row address lines, a voltage of 5 V to the column address lines Y₁₁, Y_(S21), a voltage of 0 V to the other unselected column address lines, and a voltage of 0 V to the common source line CS.

Therefore, the output node N₁₁ and the data bus SC₁ are connected to each other. If the data "0" is stored in the memory cell M_(111l), the memory cell M_(111l) is rendered nonconductive, and the voltage at the data bus SC₁ is balanced at a value of 1.5 V, which is determined by the biasing circuit of the sensing amplifier SA1. The sensing amplifier SA1 produces an output signal S_(out1) of "0".

On the other hand, if the data "1" is stored in the memory cell M_(111l), the memory cell M_(111l) is rendered conductive, and the voltage at the data bus SC₁ falls slightly from 1.5 V, which is detected by the sensing amplifier SA1. The sensing amplifier SA1 produces an output signal S_(out1) of "1".

The flash EEPROM operates in write, erase, and read modes in the manner described above.

To meet the demand in recent years for larger-capacity flash EEPROMs, there have been introduced into the market flash EEPROMs which are composed of memory cells grouped into n sectors that are individually programmable and erasable.

Flash EEPROMs composed of memory cells grouped into n sectors are advantageous for the following reasons: If memory cells whose data is to be varied exist in only one sector, the data in the other (n-1) sectors remains intact, and the sector which is composed of the memory cells that store the data to be varied is erased in its entirety at one time, after which desired data may be written successively into those memory cells in the erased sector. Consequently, the time required to write the desired data is shorter than the time needed to write data in a flash EEPROM that is not divided into sectors.

For example, if the memory cells of a 2M-bit flash EEPROM having an 8-bit data output capability are grouped into 16 sectors each having a storage capacity of 128 Kbits (16 Kbytes), data of 128 Kbits can be entirely erased at one time.

EEPROMs of increased storage capacity have problems regarding low yield as they tend to suffer malfunctions of some memory cells due, for example, to chip defects that occur when the EEPROMs are manufactured. It is known that the yield of such EEPROMs can be greatly increased when any defective memory cells are remedied. According to one attempt, a redundant cell region is provided in a chip, and upon detection of a digit line that is connected to a defective memory cell, the digit line and the defective memory cell are replaced with a digit line and a redundant memory cell which are present in the redundant cell region.

FIG. 2 of the accompanying drawings shows, in block form, memory cell matrices of an 8-bit-output flash EEPROM composed of memory cell sectors, each of which can be entirely erased at one time. The flash EEPROM has a plurality of memory cell sectors 1, . . . , n. These memory cell sectors 1, . . . , n comprise memory cell matrices MX₁₁, MX₁₂, . . . , MX₁₈, . . . , MX_(n1), MX_(n2), . . . , MX_(n8) having respective output nodes N₁₁, N₁₂, . . . , N₁₈, . . . , N_(n1), N_(n2), . . . , N_(n8), each of the memory cell matrices being identical in structure to the memory cell matrix MX₁₁ shown in FIG. 1. The memory cell sectors 1, . . . , n also include redundant memory cell regions or matrices MX_(1R), . . . , MX_(nR), respectively, having respective output nodes N_(1R), . . . , N_(nR) and redundant memory cell identical in structure to the memory cells M₁₁₁₁ -M_(1kjl) shown in FIG. 1. The redundant memory cell matrix MX_(1R) and other redundant memory cell matrices, e.g., the redundant memory cell matrix MX_(nR), are structurally identical to the memory cell matrix MX₁₁. In the memory cell sector 1, any digit line that is connected to a defective memory cell among the memory cells in the memory cell matrices MX₁₁ -MX₁₈ which cannot write, erase, or read data can be replaced with a digit line in the redundant memory cell matrix MX_(1R). In the other memory cell sectors, e.g., the memory cell sector n, any digit line that is connected to a defective memory cell among the memory cells in the memory cell matrices MX_(n1) -MX_(n8) which cannot write, erase, or read data can be replaced with a digit line in the redundant memory cell matrix MX_(nR). Those memory cell matrices other than the redundant memory cell matrices will also be referred to as main memory cell matrices or regions.

A source switch SS₁ for sector 1 is connected to a common source line CS₁ that is coupled to the sources of all the memory cells in sector 1. The source switch SS₁ can be controlled by a source switch selection line Y₂₁(S) and a signal line ER. Likewise, source switches for the other sectors are connected in the same manner as described above. For example, a source switch SS_(n) for a sector n is connected to a common source line CS_(n) that is coupled to the sources of all the memory cells in sector n. The source switch SS_(n) can be controlled by a source switch selection line Y_(2n)(S) and signal line ER.

Data buses SC₁, SC₂, . . . , SC₈ are connected to the source of the data writing NE-MOSFET Q_(Y1W) and input terminals of sensing amplifier SA1, as shown in FIG. 1. Data buses SC₁, SC₂, . . . , SC₈ are also connected through respective sector-selecting NE-MOSFETs Q_(Y211), Q_(Y212), . . . , Q_(Y218), . . . , Q_(Y2n1), Q_(Y2n2), . . . , Q_(Y2n8) to memory cell matrices MX₁₁, MX₁₂, . . . , MX₁₈, . . . , MX_(n1), MX_(n2), . . . , MX_(n8), respectively. Sensing amplifier SA1 produces output signals S_(out1), S_(out2), . . . , S_(out8) respectively with respect to data buses SC₁, SC₂, . . . , SC₈.

A data bus SC_(R) is connected to the source of the data writing NE-MOSFET Q_(Y1W) and an input terminal of sensing amplifier SA1, as shown in FIG. 1. Data bus SC_(R) is also connected through sector-selecting NE-MOSFETs Q_(Y21R), . . . , Q_(Y2nR) to redundant memory cell matrices MX_(1R), . . . , MX_(nR), respectively. Sensing amplifier SA1 produces an output signal S_(outR) with respect to data bus SC_(R).

Sector selection lines Y₂₁, . . . , Y_(2n) are connected to the gates of selector-selecting NE-MOSFETs Q_(Y211), Q_(Y212), . . . , Q_(Y218), Q_(Y21R), . . . , Q_(Y2n1), Q_(Y2n2), . . . , Q_(Y2n8), Q_(Y2nR).

The flash EEPROM shown in FIG. 2 differs from the flash EEPROM shown in FIG. 1 in that whereas data is written and read through column-address selecting NE-MOSFET NE-MOSFETs Q_(YS211), . . . , Q_(YS21j) in the write and read modes in the flash EEPROM shown in FIG. 1, data is written and read through sector-selecting NE-MOSFETs Q_(Y211), . . . , Q_(Y2n1) in the flash EEPROM shown in FIG. 2. The voltages at the nodes in the write and read modes shown in the above table apply to the flash EEPROM shown in FIG. 2 except that Y_(S21) should change to Y₂₁.

Operation of the flash EEPROM shown in FIG. 2 in sector erase mode will be described below.

It is assumed that sector 1 is selected to be erased. Signal line ER is set to a high level ("H"), source switch selection line Y₂₁(S) is set to a high level ("H"), and the other source switch selection lines Y₂₂(S) -Y_(2n)(S) are set to a low level ("L"). A voltage of 12 V is then applied to common source line CS₁, and a voltage of 0 V is applied to the other common source lines CS₂ -CS_(n). Since a voltage of 0 V is applied to all of the column now address lines at this time as shown in the above table, a high electric field is applied between the gates and sources of only the memory cells belonging to sector 1, and the data stored in the memory cells of sector 1 are erased.

While the flash EEPROM shown in FIG. 2 operates in sector erase mode as described above, all the data stored in the flash EEPROM can also be erased when all source switches SS₁ -SS_(n) are rendered active.

If it is found that the flash EEPROM shown in FIG. 2 contains a memory cell which is unable to write, erase, or read any data, then the digit line connected to the faulty memory cell can be replaced with a digit line in the redundant memory cell region or matrix.

It is assumed that a digit line D₁₈₁ in memory cell matrix MX₁₈ is to be replaced with a digit line D_(1R1) in redundant memory cell matrix MX_(1R) in sector 1.

FIG. 3 of the accompanying drawings shows a control circuit RED₁₁ for replacing digit lines D₁₁₁, D₁₂₁, . . . , D₁₈₁ in respective main memory cell matrices MX₁₁, MX₁₂, . . . , MX₁₈ with digit line D_(1R1) in redundant memory cell matrix MX_(IR) in sector 1. For the sake of brevity, the number of memory cell sectors used is assumed to be 16.

Control circuit RED₁₁ has address input terminals A₀, A₁, A₂, A₃ for supplying addresses to select sector selection lines Y_(2l), . . . , Y_(2n) shown in FIG. 2. A sector selection decoder Y-DEC-2 is supplied with the address data from address input terminals A₀, A₁, A₂, A₃. Redundancy address circuits RAC₀, RAC₁, RAC₂, RAC₃ are controlled by the addresses supplied respectively from address input terminals A₀, A₁, A₂, A₃. Redundancy address circuits RAC₀, RAC₁, RAC₂, RAC₃ each have internal fuses for storing the address of a sector to be replaced. Redundancy address circuits RAC₀, RAC₁, RAC₂, RAC₃ detect whether supplied address data agrees with the stored address data, and produce respective output signals RA₁₀, RA₁₁, RA₁₂, RA₁₃ when the supplied address data agrees with the stored address data. A redundancy selection circuit RSC stores data indicative of whether the flash EEPROM uses a redundancy memory cell matrix or not. Such data is stored by selectively melting an internal fuse in the redundancy selection circuit RSC with a control signal supplied from an external terminal D_(W). When a redundancy memory cell matrix is used, the redundancy selection circuit RSC signal RY₂₁ of "1".

Redundancy bit circuits RBC₀, RBC₁, RBC₂ each have fuses which are selectively melted by control signals supplied from respective input terminals D₀, D₁, D₂ so as to store data indicative of which output bit the substituted digital line is to be assigned to.

A redundancy access circuit RACC₁₁ produces an output signal Y_(R) of "1" when the address data supplied from the input terminals A₀, A₁, A₂, A₃ agrees with the address data stored in redundancy address circuits RAC₀, RAC₁, RAC₂, RAC₃, i.e., when the sector selected by the address data supplied from input terminals A₀, A₁, A₂, A₃ agrees with the sector whose memory cell matrix is to be replaced with the redundancy memory cell matrix. Redundancy access circuit RACC₁₁ comprises a NAND gate NAND_(R) and an inverter INV_(R).

Control circuit RED₁₁ also has a control circuit TR₁₁ which controls the transfer of the output data of the sensing amplifier that indicates a redundancy memory cell to either one of output signals D_(out1) -D_(out8). Circuit TR₁₁ comprises a decoder composed of NAND gates NAND₁ -NAND₈ coupled to respective inverters INV₁ -INV₈, and a switching section composed of switches SW₁ -SW₈.

As described above, digit line D₁₈₁ in memory cell matrix MX₁₈ is assumed to be replaced with digit line D_(1R1) in redundant memory cell matrix MX_(1R) in sector 1. In a procedure for checking the functions of the flash EEPROM, the address of sector 1 in which digit line D₁₈₁ is to be replaced with digit line D_(1R1) and data representing corresponding output bits are written by fuses. For example, it is assumed that when no data is written by the fuses, output signals "0" are produced. The fuses of redundancy address circuits RAC₀, RAC₁, RAC₂, RAC₃ do not write any data provided the address indicative of sector 1 corresponds to (RA₁₀, RA₁₁, RA₁₂, RA₁₃)=(0, 0, 0, 0), and all of the fuses of the redundancy bit circuits RBC₀, RBC₁, RBC₂ write data provided data indicative of an output bit 8 corresponds to (D₀, D₁, D₂)=(1, 1, 1).

If the fuses of redundancy bit circuits RBC₀, RBC₁, RBC₂ comprise polysilicon fuses, data may be written by melting the fuses with a laser beam or electrically melting the fuses with a high voltage applied to input terminals D₀, D₁, D₂.

The fuses may comprise memory cells identical to memory cell M₁₁₁₁ shown in FIG. 1 such that data can be electrically written thereby.

Since a redundant memory cell matrix is used, data is also written by the fuse of the redundancy selection circuit RSC.

As described above, it is determined whether or not data is to be written by the fuses in the procedure for checking the functions of the flash EEPROM. In this embodiment, if the data stored by the fuses of redundancy address circuits RAC₀, RAC₁, RAC₂, RAC₃ is designated by RAC_(f0), RAC_(f1), RAC_(f2), RAC_(f3), respectively, the stored data is (RAC_(f0), RAC_(f1), RAC_(f2), RAC_(f3))=(0, 0, 0, 0), respectively.

Inasmuch as the bit for outputting the redundancy memory cell data is an output bit 8, the fuses of redundancy bit circuits RBC₀, RBC₁, RBC₂ store data (RB₁₀, RB₁₁, RB₁₂)=(1, 1, 1), respectively. Redundancy bit circuits RBC₀, RBC₁, RBC₂ also store signals BRB₁₀, BRB₁₁, BRB₁₂, which are an inversion of the data RB₁₀, RB₁₁, RB₁₂. Therefore, this inverted data is (BRB₁₀, BRB₁₁, BRB₁₂)=(0, 0, 0). The fuse of redundancy selection circuit RSC stores the data (RY₂)=(1).

The stored data of the fuses is established as described above. In the read mode, when address data (A₀, A₁, A₂, A₃)=(0, 0, 0, 0) is supplied from address input terminals A₀, A₁, A₂, A₃, respectively, since the supplied address data (A₀, A₁, A₂, A₃) agrees with the data RAC_(f0), RAC_(f1), RAC_(f2), RAC_(f3) stored by the fuses of redundancy address circuits RAC₀, RAC₁, RAC₂, RAC₃, redundancy address circuits RAC₀, RAC₁, RAC₂, RAC₃ produce respective output signals (RA₁₀, RA₁₁, RA₁₂, RA₁₃)=(1, 1, 1, 1). Therefore, redundancy access circuit RACC₁₁ produces an output signal Y_(R) of "1," activating control circuit TR₁₁. Control circuit TR₁₁ is supplied with the data (RB₁₀, RB₁₁, RB₁₂)=(1, 1, 1) and the data (BRB.sub. 10, BRB₁₁, BRB₁₂)=(0, 0, 0) from redundancy bit circuits RBC₀, RBC₁, RBC₂. Thus, NAND gate NAND₈ produces output signal P₁₈ of "0," and inverter INV₈ produces output signal q₁₈ of "1." On the other hand, NAND gates NAND₁ -NAND₇ produce respective output signals P₁₁ -P₁₇ of "1," and inverters INV₁ -INV₇ produce respective output signals q₁₁ -q₁₇ of "0".

Switch SW₁ is arranged to transfer output signal S_(out1) as its output signal D_(out1) when output signal P₁₁ is "1" and output signal q₁₁ is "0," and to transfer output signal S_(outR) as its output signal D_(out1) when output signal P₁₁ is "0" and output signal q₁₁ is "1". Switches SW₂ -SW₈ are also similarly arranged. At this time, therefore, switch SW₈ transfers output signal S_(outR), which represents the data stored in a memory cell connected to digit line D_(1R1) in redundant memory cell matrix MX_(1R) as its output signal D_(out8). Switches SW₁ -SW₇ transfer output signals S_(out1) -S_(out7), which represent the data stored in the memory cells in the main memory cell matrices as their output signals D_(out1) -D_(out7), respectively.

The gates of the NE-MOSFETs connected to data bus SC_(R) shown in FIG. 2 are controlled such that the data to be written in the memory cell which is to be replaced will be supplied to the memory cell in the redundant memory cell matrix.

The circuit arrangement shown in FIG. 3 is required to replace a digit line in a main memory cell matrix with a digit line in a redundant memory cell matrix in one sector. If the flash EEPROM is composed of 16 memory cell sectors, it requires 16 control circuits RED₁₁ shown in FIG. 3. The total number of fuses required is 113 (7 fuses of RAC₀ -RAC₃, RBC₀ -RBC₂ ×16 sectors+1 fuse of RSC). Therefore, as the flash EEPROM increases in storage capacity and number of sectors, the number of fuses required increases, and the number of control circuit arrangements for switching between data read from the memory cells in the main memory cell matrices and data read from the memory cells in the redundant memory cell matrices, resulting in a larger flash EEPROM chip.

As described above, the conventional flash EEPROM composed of erasable memory cell sectors includes redundant memory cell matrices for the purpose of increasing the yield of flash EEPROMs, and each of the memory cell sectors has a redundant memory cell matrix including a digit line to replace a digit line connected to a memory cell that is unable to erase or read data. As a consequence, in the conventional flash EEPROM:

(1) each memory cell sector requires fuses to store data indicative of which output bit a substituted digit line is to be assigned to, and a logic circuit for decoding the stored data; and

(2) since a relatively large number of fuses are employed, a relatively long period of time is needed for the fuses to write data in the procedure for checking the functions of the flash EEPROM.

For the above reasons, the conventional flash EEPROM has been disadvantageous in that it has an increased chip size and requires a long check time, and hence, results in an increased manufacturing cost.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a semiconductor memory device which has a smaller circuit arrangement required to replace a memory cell in a main memory cell matrix with a memory cell in a redundant memory cell matrix, and which has a smaller number of fuses for storing a memory cell sector in which to replace a memory cell and fuses for storing the number of an output bit which data read from the redundant memory cell matrix is to be assigned to.

According to the present invention, there is provided a nonvolatile semiconductor memory device comprising a plurality of sectors each comprising a plurality of main memory cell regions each composed of a matrix of nonvolatile memory cells, control means for writing data in, erasing data from, and reading data from the nonvolatile memory cells in each of the sectors, at least one redundant memory cell region composed of a matrix of nonvolatile memory cells, and replacing means for replacing one of the nonvolatile memory cells in any one of the sectors selected by addressing and found to be defective with one of the nonvolatile memory cells within that at least one redundant memory cell region.

The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram, partly in block form, of a conventional semiconductor memory device,

FIG. 2 is a block diagram of a conventional flash EEPROM,

FIG. 3 is a block diagram of a control circuit in the flash EEPROM shown in FIG. 2 for replacing a digit line in a main memory cell matrix with a digit line in a redundant memory cell matrix,

FIG. 4 is a block diagram of semiconductor memory device in the form of a flash EEPROM according to a first embodiment of the present invention,

FIG. 5 is a block diagram of a control circuit in the flash EEPROM shown in FIG. 4, for replacing a digit line in a main memory cell matrix with a digit line in a redundant memory cell matrix, and

FIG. 6 is a block diagram of a semiconductor memory device in the form of a flash EEPROM according to a second embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in FIG. 4, a semiconductor device according to a first embodiment is in the form of an 8-bit-output flash EEPROM composed of 16 memory cell sectors each erasable in its entirety at one time. Each of the memory cell sectors comprises a plurality of memory cell regions or matrices. Parts shown in FIG. 4 which are identical to those shown in FIG. 2 are denoted by identical reference characters, and will not be described in detail below.

In FIG. 4, the flash EEPROM has first and second redundant memory cell matrices MX_(R1) and MX_(R2) which can be selected respectively by sector selection lines Y_(2R1) and Y_(2R2). As with sector selection lines Y₂₁, . . . , Y₂₁₆, in write mode, a voltage of 12 V is applied to sector selection lines Y_(2R1) and Y_(2Rn) when redundant memory cell matrices MX_(R1) and MX_(R2) are selected, and a voltage of 0 V is applied to sector selection lines Y_(2R1) and Y_(2Rn) when redundant memory cell matrices MX_(R1) and MX_(R2) are not selected, as indicated in the above table at Y_(S21). In erase mode, a voltage of 0 V is applied sector selection lines Y_(2R1) and Y_(2Rn). In read mode, a voltage of 5 V is applied to sector selection lines Y_(2R1), Y_(2Rn) when redundant memory cell matrices MX_(R1), MX_(R2) are selected, and a voltage of 0 V is applied to sector selection lines Y_(2R1) and Y_(2Rn) when redundant memory cell matrices MX_(R1) and MX_(R2) are not selected. Sector-selecting NE-MOSFETs Q_(Y2R1) and Q_(Y2R2) control the selective connection of output nodes N_(R1) and N_(R2) of redundant memory cell matrices MX_(R1) and MX_(R2) to redundancy data bus SC_(R). Source switches SS_(R1) and SS_(R2) are connected respectively to the first and second redundant memory cell matrices MX_(R1) and MX_(R2). More specifically, source switches SS_(R1) and SS_(R2) have respective output terminals CSR1 and CSR2 connected to the sources of all of the memory cells in first and second redundant memory cell matrices MX_(R1) and MX_(R2). Source switch SS_(R1) can be controlled by a source switch selection line Y_(2R1) (S) and the signal line ER. Likewise, source switch SS_(R2) can be controlled by a source switch selection line Y_(2R2) (S) and the signal line ER.

The first redundant memory cell matrix MX_(R1) operates as a common redundant memory cell matrix for memory cell sectors 1-8, and the second redundant memory cell matrix MX_(R2) operates as a common redundant memory cell matrix for memory cell sectors 9-16. Basically, the flash EEPROM according to the first embodiment differs from the conventional flash EEPROM shown in FIG. 2 with respect to the provision of the first and second redundant memory cell matrices, MX_(R1) and MX_(R2), and the associated circuits and components, as described above.

It is assumed that the flash EEPROM shown in FIG. 4 contains a memory cell connected to digit line D₁₈₁ in memory cell matrix MX₁₈ in memory cell sector 1, and that memory cell is unable to write, erase, or read and data. A process for replacing digit line D₁₈₁ connected to the fault memory cell with a digit line D_(R1) in the first redundant memory cell matrix, MX_(R1), will be described below.

FIG. 5 shows control circuit RED₁ for replacing digit lines in the respective main memory cell matrices in memory cell sectors 1-8 with a digit line in the first redundant memory cell matrix, MX_(R1). Parts shown in FIG. 5 which are identical to those shown in FIG. 3 are denoted by identical reference characters and will not be described in detail below.

Control circuit RED₁ has address input terminals A₀, A₁, A₂, A₃ which are supplied with addresses to decode sector selection lines Y₂₁, . . . , Y₂₁₆ shown in FIG. 4. A sector selection decoder Y-DEC-21 is supplied with the address data from address input terminals A₀, A₁, A₂, A₃. Redundancy address circuits RAC₀, RAC₁, RAC₂, RAC₃ each have internal fuses for storing the address of a sector to be replaced. Redundancy address circuits RAC₀, RAC₁, RAC₂, and RAC₃ produce respective output signals RA₁₀, RA₁₁, RA₁₂, and RA₁₃ when the supplied address data agrees with the stored address data in write and read modes. A redundancy selection circuit RSC has an internal fuse which is selectively melted by a control signal from an external terminal D_(W) to store data indicative of whether the flash EEPROM uses a redundancy memory cell matrix or not. When a redundancy memory cell matrix is used, the redundancy selection circuit RSC produces an output signal RY₂₁ of "1".

As with input terminals D₀, D₁, and D₂ shown in FIG. 3, input terminals D₀, D₁, and D₂ supply control signals respectively to redundancy bit circuits RBC₀, RBC₁, and RBC₂ that each have fuses that are selectively melted by the supplied control signals to store data indicative of which output bit the substituted digit line is to be assigned to.

A redundancy access circuit RACC₁ produces an output signal Y_(2R1) of 5 V in read mode and an output signal Y_(2R1) of 12 V in write mode when the address data supplied from input terminals A₀, A₁, A₂, and A₃ agrees with the address data stored in redundancy address circuits RAC₀, RAC₁, RAC₂, and RAC₃. Redundancy access circuit RACC₁ produces an output signal Y_(2R1) of 0 V when any one of the output data RA₁₀ -RA₁₃, and RY₂₁ is "0". Redundancy access circuit RACC₁ comprises NAND gate NAND_(R1) and inverter INV_(R1). Output signal Y_(2R1) is supplied to sector selection line Y_(2R1) shown in FIG. 4 to control the gate of NE-MOSFET QY_(2R1).

Control circuit RED₁ also has control circuit TR₁ which controls the transfer of output data S_(outR) of the sensing amplifier that has read a memory cell in first redundant memory cell matrix MX_(R1) to one of output signals D_(out1) -D_(out8), as with control circuit TR₁₁ shown in FIG. 3. Output signals D_(out1) -D_(out8) are supplied to the input terminals of respective output buffers (not shown).

As described above, digit line D₁₈₁ in memory cell matrix MX₁₈ in memory sector 1 is assumed to be replaced with digit line D_(R1) in first redundant memory cell matrix MX_(R1). In a procedure for checking the functions of the flash EEPROM, the address of sector 1 is written by the fuses of redundancy address circuits RAC₀ through RAC₃. For example, if the data stored by the fuses of redundancy address circuits RAC₀, RAC₁, RAC₂, and RAC₃ are indicated respectively by RAC_(f0), RAC_(f1), RAC_(f2), and RAC_(f3), this stored data is represented by (RAC_(f0), RAC_(f1), RAC_(f2), RAC_(f3))=(0, 0, 0, 0). The fuse of redundancy selection circuit RSC stores data and produces an output signal RY₂₁ of "1".

The data indicating an output bit 8 is stored by the fuses of redundancy bit circuits RBC₀, RBC₁, and RBC₂. Thus, redundancy bit circuits RBC₀, RBC₁, and RBC₂ store data (RB₁₀, RB₁₁, RB₁₂)=(1, 1, 1) and inverted data (BRB₁₀, BRB₁₁, BRB₁₂)=(0, 0, 0, respectively.

With the data being thus stored by the fuses in control circuit RED₁, when sector 1 is selected, the flash EEPROM operates in write, erase, and read modes as follows, depending on whether the supplied address data corresponds or not to a digit line to be replaced.

1. Write mode

(1) When the supplied address data does not correspond to a digit line to be replaced, the nodes shown in FIG. 4 are controlled as indicated above in the table, and supplied data is stored in a memory cell indicated by the supplied address data in a main memory cell matrix. For details, reference should be made to the foregoing description with reference to FIGS. 2 and 3.

(2) When the supplied address data corresponds to a digit line to be replaced, a voltage of 12 V is applied to both sector selection lines, Y₂₁ and Y_(2R1), rendering NE-MOSFETs Q_(Y2R1), Q_(Y211), . . . , Q_(Y218) conductive. Since an output bit to be replaced is an output bit 8, the same data as the data supplied to data bus SC₈ is supplied to data bus SC_(R). For example, when a memory cell connected to digit line D₁₈₁ is selected and data is to be written in the selected memory cell, a voltage of (Vpp-VTN) is applied to data bus SC₈. At this time, the voltage of (Vpp-VTN) is applied to data bus SC_(R). Therefore, a voltage of (Vpp-VTN) is applied to digit line D_(R1) in redundant memory cell matrix MX_(R1), and the data is stored in the memory cell which is selected by a column address line. Source switch SS_(R1) applies an output signal CSR₂ of 0 V.

2. Sector erase mode

(1) For erasing sectors other than sector 1, a voltage of 12 V is applied to terminal PP, and a voltage of 0 V is applied to all of the column address lines. To erase sector 2, a voltage of 0 V is applied to sector selection line Y₂₂. At the same time, source switch selection line Y₂₂(S) is supplied with a signal "1", activating source switch SS₂. Source switch SS₂ produces an output signal of 12 V, thereby erasing all of the memory cells in sector 2.

(2) To erase sector 1, source switch selection lines Y₂₁(S) and Y_(2R1)(S) are supplied with a signal "1", activating source switches SS₁ and SS_(R1). Source switches SS₁ and SS_(R1) produce an output signal of 12 V. A voltage of 0 V is imposed on sector selection line Y₂₁ and sector selection line Y_(2R1) to select first redundant memory cell matrix MX_(R1). Therefore, all of the memory cells in sector 1 and all of the memory cells in first redundant memory cell matrix MX_(R1) are erased. The net result is that the memory cells in first redundant memory cell matrix MX_(R1) are controlled as if they were contained in sector 1.

3. Read mode

(1) When a memory cell is selected that is connected to a digit line other than D₁₈₁, the digit line to be replaced, a voltage of 5 V is applied to terminal PP, and a voltage of 5 V is applied to the selected column address line and the selected sector selection line. The output signal from the selected memory cell matrix is supplied to the corresponding data bus, reading the data from the selected memory cell in the manner described with reference to the prior art.

(2) When a memory cell is selected that is connected to digit line D₁₈₁ that is to be replaced, since the stored data is established such that (RA₁₀, RA₁₁, RA₁₂, RA₁₃)=(1, 1, 1, 1), (RY₂₁)=(1), (RB₁₀, RB₁₁, RB₁₂)=(1, 1, 1), and (BRB₁₀, BRB₁₁, BRB₁₂)=(0, 0, 0), redundancy access circuit RACC₁ produces an output signal Y_(2R1) of 5 V. In control circuit TR₁, NAND gate NAND₈ produces an output signal P₁₈ of "0," inverter INV₈ produces an output signal q₁₈ of "1," NAND gates NAND₁ -NAND₈ each produce output signals P₁ -P₁₇ of "1," and inverters InV₁ -INV₁₇ each produce output signals q₁ -q₁₇ of "0." Therefore, control circuit TR₁ transfers the data S_(out1) -S_(out7) to output signals D.sub. out1 -D_(out7) and also transfers the data S_(outR) to output signal S_(out8).

Since NE-MOSFET Q_(Y2R1) shown in FIG. 4 is rendered conductive at this time, the data stored in the memory cell that is connected to digit line D_(R1) and selected by the column address line appears on redundancy memory cell data bus SC_(R). The sensing amplifier coupled to data bus SC_(R) then reads the data stored in the memory cell, and output signal S_(outR) is delivered as output signal D_(out8), replacing output signal S_(out8).

In the above example, a memory cell connected to digit line D₁₈₁ in sector 1 is replaced with a memory cell connected to digit line D_(R1) in first redundant memory cell matrix MX_(R1), and the data from the substituted memory cell is assigned to an output bit 8. However, the fuses of redundancy address circuits RAC₀, RAC₁, RAC₂, and RAC₃ may be selectively melted to store data representing any sector ranging from sectors 1 to 8, and the fuses of redundancy bit circuits RBC₀, RBC₁, and RBC₂ may be selectively melted to store data indicative of any bit ranging from output bits 1 to 8 which the data stored in the substituted memory cell is to be assigned to.

The circuit arrangement shown in FIG. 5 is required to replace a digit line in a sector ranging from sectors 1 to 8 with a digit line in first redundant memory cell matrix MX_(R1). The flash EEPROM shown in FIG. 4 also requires a circuit for replacing a digit line in a sector ranging from sectors 9 to 16 with a digit line in a second redundant memory cell matrix MX_(R2). Consequently, the flash EEPROM shown in FIG. 4 needs two control circuits RED₁ and a redundancy selection circuit RSC, shown in FIG. 5. The overall circuit arrangement for replacing digit lines connected to defective memory cells in the main memory cell matrices with digit lines in the redundancy memory cell matrices is much smaller in scale than the conventional circuit arrangement shown in FIG. 2. The total number of fuses required is 15 (7 fuses of RAC₀ -RAC₃, RBC₀ -RBC₂ ×2 control circuits RED₁ +1 fuse of RSC), which is far less than the total number of fuses used in the conventional flash EEPROM.

FIG. 6 shows a flash EEPROM according to a second embodiment of the present invention. Parts shown in FIG. 6 which are identical to those shown in FIG. 4 are denoted by the same reference characters and will not be described in detail below.

The control circuit means for the second embodiment (FIG. 6) is similar to the control circuit means shown in FIG. 5 for the first embodiment (FIG. 4), except that while a switch SW₁ among the switches SW₁ -SW₈ in the first embodiment (FIG. 4) outputs a signal Sout₁ or Sout_(R) on output Dout₁, a switch SW₁ of switches SW₁ -SW₈ in the second embodiment (FIG. 6) outputs any one of the signals Sout₁, Sout_(R1) and Sout_(R2) on output Dout₁. Other construction details of the control circuit means in the second embodiment (FIG. 6) are the same as the control circuit means shown in FIG. 5.

The flash EEPROM shown in FIG. 6 differs from the flash EEPROM shown in FIG. 4 in that first and second redundancy memory cell matrices MX_(R1) and MX_(R2) are associated with redundancy memory cell data buses SC_(R1) and SC_(R2), respectively. NE-MOSFETs Q_(Y2R1) and Q_(Y2R2) for writing data and the sensing amplifier are connected to redundancy memory cell data buses SC_(R1) and SC_(R2). In the first embodiment, it is impossible to simultaneously replace memory cells in two sectors among sectors 1 through 8 which fail to write, erase, or read data. For example, a memory cell connected to digit line D₁₁₁ and a memory cell connected to digit line D₂₁₁ cannot be simultaneously replaced with memory cells in the redundant memory cell matrix. According to the second embodiment, however, since first and second redundancy memory cell matrices MX_(R1) and MX_(R2) are associated with redundancy memory cell data buses SC_(R1) and SC_(R2), respectively, digit lines D₁₁₁ and D₂₁₁, for example, may be replaced with digit lines D_(R1) and D_(R2), respectively, in first and second redundancy memory cell matrices MX_(R1) and MX_(R2). As a consequence, defective memory cells can be remedied more successfully in the flash EEPROM according to the second embodiment than in the flash EEPROM according to the first embodiment.

The control circuit arrangement required to replace memory cells in the main memory cell matrices with memory cells in the redundant memory cell matrices according to the second embodiment is more complex than the control circuit arrangement shown in FIG. 5 according to the first embodiment. Specifically, it is necessary for the control circuit arrangement according to the second embodiment to effect a control process to determine which data read from a memory cell in the first or second redundancy memory cell matrix is to replace the data in a defective memory cell in the main memory cell matrix. Such a control process will not be described below as it can be readily understood from FIG. 5.

Although certain preferred embodiments of the present invention have been shown and described in detail, it should be understood that various changes and modifications may be made therein without departing from the scope of the appended claims. 

What is claimed is:
 1. A nonvolatile semiconductor memory device comprising:a plurality of sectors each comprising a plurality of main memory cell regions each composed of a matrix of nonvolatile memory cells; control means for writing data in, erasing data from, and reading data from said nonvolatile memory cells in each of said sectors; at least one redundant memory cell region composed of a matrix of nonvolatile memory cells; replacing means for replacing any one of said nonvolatile memory cells which is selected by addressing in any one of said sectors and when the addressed cell is found to be defective, said replacing being with one of the nonvolatile memory cells and in said at least one redundant memory cell region; and voltages of sources of memory cells constituting each sector being controlled and driven by a circuit means; sources of memory cells in said redundant memory cell regions being controlled and driven by an independent circuit means which is other than said circuit means which controls and drives the voltages of the sources of memory cells constituting each sector.
 2. A nonvolatile semiconductor memory device according to claim 1, wherein said nonvolatile memory cells in said at least one redundant memory cell region are structurally and characteristically identical to said nonvolatile memory cells in said main memory cell regions.
 3. A nonvolatile semiconductor memory device according to claim 1, wherein said sectors are divided into a plurality of groups, and said at least one redundant memory cell region being a plurality of redundant memory cell regions each composed of a matrix of nonvolatile memory cells, said redundant memory cell regions being associated with said groups, respectively, of said sectors, said replacing means comprising means for replacing the defective and selected nonvolatile memory cell with the nonvolatile memory cell in a redundant memory cell region which is associated with a group of the sector which contains said defective and selected nonvolatile memory cell.
 4. A nonvolatile semiconductor memory device according to claim 3, wherein said replacing means comprises means for replacing the defective and selected nonvolatile memory cell with one of the nonvolatile memory cells in any one of the redundant memory cell regions which is associated with another one of said groups of the sectors other than the group that contains the defective and selected nonvolatile memory cell.
 5. A nonvolatile semiconductor memory device according to claim 1, wherein said replacing means comprises means for replacing a digit line connected to a drain of the defective and selected nonvolatile memory cell with a digit line connected to a drain of the nonvolatile memory cell in said at least one redundant memory cell region.
 6. A nonvolatile semiconductor memory device according to claim 1, further including:k column address lines, l row address lines, m data buses, n sector selection lines, a redundant memory cell region selection line, and a redundant memory cell data bus, wherein said nonvolatile memory cells of said main memory cell regions of each sector have drains connected to digit lines, gates connected to said row address lines, and sources connected in common to a respective source terminal of each sector, said main memory cell regions include column address selecting transistors connected between said digit lines and output nodes of the memory cell regions, said column address selecting transistors having gates for respective column address lines; said nonvolatile semiconductor memory device further including sector selecting transistors connected respectively between said output nodes and data buses corresponding to said output mode and having respective gates connected to respective said sector selection lines; wherein said nonvolatile memory cells of said redundant memory cell region have drains connected to redundant memory cell digit line, gates connected to said row address lines, and sources connected in common to a source terminal other than said source terminals of the sectors, said redundant memory cell region including column address selecting transistors connected between said redundant memory cell digit line and an output node of the redundant memory cell region and said column address selecting transistors having gates connected respectively to column address lines; and further including a redundant memory cell region selecting transistor connected between said output node of the redundant memory cell region and said redundant memory cell data bus and having a gate connected to said redundant memory cell region selection line.
 7. A nonvolatile semiconductor memory device according to claim 1, wherein said replacing means comprises:redundancy address circuits having fuses for storing address data of one of the sectors which contains the defective and selected nonvolatile memory cell to be replaced, said redundancy address circuits comprising means for supplying address data to decode sector selection lines and detecting whether the supplied address data agrees with stored address data in write and read modes; redundancy bit circuits having fuses for storing data indicative of which output bit the nonvolatile memory cell in the redundant memory cell region to replace the defective and selective nonvolatile memory cell is to be assigned to; a redundancy access circuit for selectively applying predetermined voltages respectively in read mode and write mode to a line for selecting the nonvolatile memory cell in the redundant memory cell region which is to replace the defective and selected nonvolatile memory cell when the supplied address data agrees with stored address data in said redundancy address circuits; and a control circuit for transferring stored data read from the nonvolatile memory cell in the redundant memory cell region to replace the defective and selected nonvolatile memory cell to an output buffer circuit corresponding to said output bit whose data is stored by said redundancy bit circuits. 